I don't know about the part numbers, but my 1980 450sl exhibits similar characteristics. I've recently figured out that my SL is truly a warm weather car, as the cruise control has suddenly decided to work again. ALthough, it still does some sudden moves where it quits or loses speed. My experience has been that the system is not real smooth, but I may (and most likely do) have some part(s) not functioning properly.

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Chris Meyer
1980 450 SL (99k miles)
2003 Honda Odyssey (Family Hauler)
2006 BMW 330i (daily driver)